Cathodic electrodeposition coating can apply a coating film onto fine portions of metal base materials with curves and bag portions, formed by fold-processing metal plates, and also plural curves such as connecting portions between metal plates. The cathodic electrodeposition coating can also form a coating film automatically and continuously, and therefore, has been widely practically applied as a method of base coating for large-size metal base materials with plural curves and bag portions such as car bodies in particular. The cathodic electrodeposition coating is performed by immersing a material to be coated into a cathodic electrodeposition coating composition as a negative electrode and applying a voltage thereto. A coating film is deposited in the process of the cathodic electrodeposition coating by an electrochemical reaction so that a component in the electrodeposition coating composition moves to the surface of the material to be coated by cataphoresis and a cathodic electrodeposition coating film is deposited on the surface of the material to be coated. Since, the deposited coating film has an insulating property, electric resistance of the coating film increases as the deposition of the coating film progresses in the process of the cathodic electrodeposition coating and the thickness of the coating film increases. As a result, the deposition of the coating film decreases at the site and the deposition of the coating film begins alternatively at undeposited sites. In this way, the coating film deposits sequentially at undeposited sites to thereby complete the electrodeposition coating film over the entire material to be coated. The property to form a continuous electrodeposition coating film by way that an insulating coating film is sequentially deposited, at undeposited sites of a metal base material of a material to be coated is referred to as “uniformity” in this specification. The cathodic electrodeposition coating sequentially forms an insulating coating film on the surface of a material to be coated as described above, and therefore, theoretically has an infinite uniformity and can form a uniform coating film on all portions of materials to be coated. However, the uniformity of electrodeposition coating film tends to degrade considerably in cases where the electric resistance of the coating film does not increase for some reason even when the coating film is deposited on the surface of material to be coated. Consequently, the nonuniformity generated in film thickness significantly affects the corrosion resistance etc. When the cathodic electrodeposition coating film is applied to metal base materials, surface treatment is typically applied in order to improve various properties such as corrosion resistance and coating adhesion. Chromic phosphate based surface treatment compositions, which have heretofore been employed for surface treatment in view of improvement in coating adhesion and corrosion resistance, have recently been pointed out for their environmental impact due to the hazardous properties of chromium. Accordingly, zinc phosphate based surface treatment compositions have been employed as a surface treatment agent containing no chromium (e.g., see Patent Document 1). However, the zinc phosphate based surface treatment compositions have a high metal ion content as well as a high acid content and exhibit very strong reactivity and thus are undesirable in view of economy and workability such as expensive wastewater treatment. In addition, during chemical conversion treatment of metal using zinc phosphate based surface treatment agents, water-insoluble salts are generated and separate out as a deposit inside chemical conversion treatment baths. Such a deposit is referred to as “sludge” in general and is problematic in terms of higher cost for removal and disposal of the sludge. Furthermore, phosphate ion may possibly provide an environmental load such as nutrient enrichment of rivers and oceans. Additionally, surface conditioning is necessary for surface treatment by zinc phosphate based surface treatment compositions and is problematic in terms of longer processes of surface treatment. Surface treatment compositions including metal surface treatment agents of zirconium and/or titanium compounds are publicly known as substitutes for chromic phosphate based or zinc phosphate based surface treatment compositions. For example, Patent Document 2 discloses an aqueous surface treatment liquid for surface-treating each independently or at least two simultaneously of metal materials selected from iron materials, zinc materials, aluminum materials, and magnesium materials, in which the surface treatment liquid for metal surface is characterized in containing at least one compound selected from zirconium compounds and titanium compounds in an amount of 5 ppm to 5000 ppm as the metal element and also free fluorine ion in an amount of 0.1 ppm to 100 ppm, and has a pH of 2 to 6. In accordance with the surface treatment liquid, a surface treatment film with superior corrosion resistance after coating can be allegedly deposited on a metal surface of each independently or two to four simultaneously of iron materials, zinc materials, aluminum materials, and magnesium materials using a treatment bath containing no environmental harmful component without generating the sludge, which has been impossible in the prior art. Furthermore, Patent Document 3 discloses a pretreatment method for coating to treat a material to be treated by a chemical conversion treatment agent to form a chemical conversion film, in which the pretreatment method for coating is characterized in that the chemical conversion treatment agent contains at least one selected from the group consisting of zirconium, titanium, and hafnium; fluorine, and at least one selected from the group consisting of amino group-containing silane coupling agents, hydrolysates thereof, and polymers thereof. In accordance with the pretreatment method for coating, the environmental load may be lower due to employing no zinc phosphate based treatment agent and a chemical conversion film can be formed with superior film adhesion even onto iron base materials to which pretreatment had been heretofore inadequate using chemical conversion treatment agents containing zirconium.
Patent Document 4 discloses a pretreatment method for coating to form a chemical conversion film on surface of car bodies of material to be treated prior to electrodeposition coating, in which the pretreatment method for coating is characterized in applying a degreasing treatment and a cleaning treatment to the car bodies, and applying a chemical conversion treatment using a chemical conversion treatment liquid, followed by warming the car bodies to the temperature equivalent to that of the electrodeposition liquid during the electrodeposition coating. In accordance with the pretreatment method for coating, allegedly, electrodeposition uniformity can be improved and quality of the coating film can be improved. Patent Document 5 discloses a method of pretreating a surface of aluminum or alloy thereof prior to another stable corrosion-prevention chemical conversion treatment, preferably, chromate salt treatment, chromium non-containing chemical conversion treatment by a reactive organic polymer and/or a compound of titanium, zirconium, and/or hafnium elements, or phosphate treatment by an acidic zinc-containing phosphate treatment bath, in which the method is characterized in that the surface is brought into contact with an aqueous treatment solution which contains a fluoro complex of boron, silicon, titanium, zirconium or hafnium elements, each independently or a mixture thereof, in an amount of 100 mg/L to 4000 mg/L, preferably 200 mg/L to 2000 mg/L as the concentration of total fluoro anion and has a pH value of 0.3 to 3.5, preferably 1 to 3; and a method is disclosed as one embodiment thereof in which the treatment solution, having a temperature of 15° C. to 60° C., is applied to aluminum surface by a spray, immersion, or non-rinsing process, and the treated aluminum surface is dried at a temperature of 40° C. to 85° C. In accordance with the method, allegedly, contact resistance of the metal surface can be made uniform and weld can be made uniform at resistance welding.
[Patent Document 1] Japanese Patent No. 3088623
[Patent Document 2] Japanese Unexamined Patent Application, First Publication No. 2004-190121
[Patent Document 3] Japanese Unexamined Patent Application, First Publication No. 2004-218070
[Patent Document 4] Japanese Unexamined Patent Application, First Publication No. 2006-183128
[Patent Document 5] Japanese Unexamined Patent Application, First Publication No. 08-510505